Vacuum forming is a well known process, particularly in the automotive industry. For example, a thermoplastic material (e.g., polyvinyl and polyurethane flexible sheeting) can be laminated onto automobile components such as consoles, instrument panels, arm rests, door panels and other interior surfaces using vacuum forming. The thermoplastic material has an outer side, which typically provides color and texture to the resulting laminated substrate, and an underside which can further include a primer layer for enhanced adhesion to the substrate. Common substrates typically include a variety of known materials, such as acrylonitrile-butadiene-styrene terpolymer (ABS) or fiberboard. The process typically includes coating the substrate with a laminating adhesive, heating the thermoplastic material past its softening point, and draping the thermoplastic material onto the substrate. A vacuum is then applied through the substrate to pull the thermoplastic material onto the contours of the substrate, thereby forming a conformal layer of the thermoplastic material over at least a portion of the substrate.
Use of organic solvent-based adhesives in vacuum forming is common in the industry. However, emission of organic solvents to the atmosphere can be a concern, and reduction of the volatile organic content of adhesive compositions is very desirable to comply with government regulations. Such adhesives also can present work hazards typically associated with organic solvents. Many of the known adhesive compositions are provided in the form of a two-pack system. Exemplary of such two-pack solvent-based compositions are those known in the art in which one pack is based on an aqueous aromatic polyurethane emulsion or dispersion, and another pack is based on a water-dispersible crosslinking agent such as an unblocked isocyanate. The two packs must be stored separately and mixed only just prior to application. Once admixed, such compositions are stable for only several hours, and within several hours recognizable performance decreases are observed.
In view of the foregoing discussion, single package water-based vacuum-forming laminating adhesive compositions can provide obvious advantages over the two-package solvent-based compositions. Such aqueous adhesive compositions are known. For example, known are stable aqueous adhesive compositions that include a vinyl acetate polymer, an ionic water-dispersed polyurethane, and an aziridine. Also known in the art are water-based vacuum-forming laminating adhesive compositions that include an ionic water-dispersed polyurethane formed by chain extending an isocyanate functional prepolymer with a polyamine and a ketimine, a carbodiimide and/or an aziridine and, optionally a vinyl acetate polymer. Such water-based adhesive compositions provide excellent adhesion of thermoplastic materials to conventional rigid substrates such as ABS and fiberboard.
More recently, there has been an attempt to replace the aforementioned ABS and fiberboard rigid substrates with substrates comprising materials which are more readily recyclable such as rigid thermoplastic polyolefinic materials which are lower in surface energy than ABS, for example polypropylene. However, conventional water-based laminating vacuum-formable adhesive compositions such as those described above often do not provide sufficient adhesion of the thermoplastic material to such polyolefin substrates.
Thus, it would be desirable to provide an aqueous-based, vacuum-formable laminating adhesive composition which provides excellent adhesion of thermoplastic materials to rigid polyolefin substrates, while maintaining heat and humidity resistance properties.